Stuttered Swallowing and the Insular Cortex

There is a very interesting and sophisticated case-report from a group in Canada locating a specific function within the insular cortex. The group (Soros et al) detail their findings in a paper titled ‘Stuttered swallowing: Electric stimulation of the right insula interferes with water swallowing. A case report’ and freely available as an Open Access article with a Creative Commons Attribution license 2.0 here. The insular cortex is a part of the brain which has been associated both with visceral sensations and emotions. Indeed a central role for the insular cortex in human emotions has been developed by both Antonio Damasio and by A Craig building on Damasio’s work (see here and here for further reading).

The case report is about a 24 year old man with a history of intractable epilepsy. He is diagnosed with a ganglioglioma which is succesfully resected leading to a remission in the seizure activity. Prior to the resection however the neurosurgical team carry out interaoperative recording of activity in different parts of the brain and correlate this with functional activities.

Location of Ganglioglioma in Right Posterior Insular Cortex from (Soros et al, 2011)

In cases of resection the neurosurgeon will try to distinguish non-functional or pathological tissue from functional tissue. In this case, the patient remains conscious during the intraoperative recording and was able to swallow a glass of water. It was during the stimulation of the inferior posterior Insular Cortex that an irregular pattern of swallowing was noted by the researchers. They used a laryngeal muscle sensor which enabled them to describe the swallowing actions. They found both a delay in the swallowing after the stimulation and the patient described a ‘stuttered swallowing’. The swallowing movements with stimulation of two parts of the insular cortex are presented in the second diagram in the paper which shows the delayed swallowing with the stimulation of the inferior posterior Insular Cortex.

There is a continuous water infusion during the stimulation period and thus the main variable of change is assumed to be the electrical stimulation leading to the researchers concluding that there is a direct causal chain between electrical activity in the inferior posterior Insular Cortex (IPIC) and an alteration in swallowing movements. Interpreting this, it does not mean that there is necessarily a simple motor function of the IPIC but that it would be more sensible to suppose that some type of ‘integration’ activity occurs there. In other words there is an integration of visceral sensation  provided by the water infusion with motor commands. For example a summation of activity in the IPIC passing a threshold would trigger a motor activity either by inhibiting a tonic inhibition of motor activity or else influencing another motor command centre (most likely the motor cortex) with which it has efferent connections.

So how might all of this relate to the emotions? There are various ways. Gulping for instance is associated with fear. This is in effect a swallowing activity. Although this is a rather superficial association it is relevant to the James-Lange theory of emotions which posits a direct relationship between visceral sensation and subsequent emotional states. The relationship can be bidirectional and it is tempting to suppose that the insular cortex might be relative imprecise in its ability to segregate visceral and emotional data. Perhaps it consists of a large number of overlapping maps which produce unusual artefacts as in the case of gulping with fear (visceral reflexes possibly triggered by insular activity). While there are many difficulties in assigning functions to a specific area within the brain, cases such as this provide useful information which can be contextualised using the works of Damasio and Craig and can also contribute to valid models.

 

References

Peter Sörös, Faisal Al-Otaibi, Savio WH Wong J Kevin Shoemaker, Seyed M Mirsattari, Vladimir Hachinski, and Ruth E Martin.Stuttered swallowing: Electric stimulation of the right insula interferes with water swallowing. A case report. BMC Neurology. 2011; 11: 20.

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